1. Field of the Invention
The present invention relates to a process for efficiently producing fine flaky alumina particles suitable as a raw material for ceramics, a pigment for paint, etc., through a hydrothermal treatment.
Furthermore, the present invention relates to an alumina-based plastic material using an alumina powder consisting of the fine flaky alumina particles obtained by the above-mentioned process in which the alumina powder develops plasticity when kneaded with water.
2. Description of the Related Art
An alumina powder which is a representative raw material for fine ceramics has been widely used as a material having excellent heat resistance, electrical insulating properties, mechanical strengths and other properties in the field of structural components, electronic components, etc. In the molding of the alumina powder, however, no clay-like plasticity is developed through mere kneading of the alumina powder with water when the molding is conducted through the utilization of the plasticity. For this reason, in order to obtain the plasticity necessary for the molding, the plastic molding has been conducted after the incorporation of various molding assistants, for example, plastic clay and a large amount of an organic binder in the alumina powder. When these molding assistants are used in large amounts, there occur such problems that degreasing becomes necessary in the subsequent step, the molding density lowers and further the sinter becomes heterogeneous.
On the other hand, kaolinite clay, which has been used as the raw material for ordinary potteries, has itself a large plasticity. Motoki, Okuda, et al. (Yogyo Genryo, the fourth series, 99-106 (1952); Nendo Kagaku, 8, 14-23 (1968)) describe that factors for developing the plasticity are that (a) particles are fine and in a near colloidal region, (b) the form of the particle is flat, and (c) a water film formed with water (by a hydrogen bond) on the surface of the particle is thick. They, however, point out also that there remain many problems open on the development of the plasticity.
Various processes for producing fine alumina particles used in the above-mentioned fine ceramics are known in the art. In general, fine alumina particles of the order of submicron (i.e., 1.0 .mu.m or less) have been produced by pulverization through the use of mechanical means such as a ball mill. In this case, although fine particles can be obtained, no fine particles in the flaky form can be produced. Bayer's process, which is known as a process for producing alumina particles on a commercial scale, has a problem that not only alumina particles having a high purity are difficult to produce by this process but also the particles produced thereby are liable to become spherical.
Known processes for producing flaky alumina particles include a process disclosed in Japanese Patent Publication No. 35-6977, which comprises adding a mineralizer such as aluminum fluoride in the calcination step. Also known is a process described in Japanese Patent Publication Nos. 37-7750 and 39-13465, which comprises conducting hydrothermal synthesis to produce alumina particles.
Among the above-described processes for producing flaky alumina particles, the process wherein a mineralizer is added has problems on the preparation of fine particles and the service life of the apparatus. Also the hydrothermal synthesis process has a problem on the preparation of fine particles because, as is apparent from the description of the above-described publications, the particle size is of the order of from several microns to several hundred microns.